In recent years, plastic waste disposal has become a problem. Procedures mainly used for plastic waste disposal are incineration and reclamation. However, incineration is disadvantageous in that it accelerates global warming, while reclamation suffers from problems such as lack of landfills for reclamation. For example, polyurethanes are consumed all over the world at a rate of about 6,000,000 tons per year and in Japan at a rate of about 550,000 tons per year. Among them, foam cushioning materials made of polyurethanes are used on a massive scale as heat insulators, e.g., for refrigerators because of their excellent heat insulation properties. At present, polyurethane waste is often disposed of in landfills as noncombustible garbage, but problems are associated with its disposal, such as lack of landfills, and environmental pollution. While microbial biodegradation can be presented as a preferred disposal technique from the viewpoint of protection of the natural environment, a problem exists in that polyurethanes are not biodegradable.
Polyurethane contains urethane bonds together with ester or ether bonds in its molecule, and degradation proceeds through cleavage of these bonds. There are some reports of ester bonds in polyol units being cleaved by fungi and/or bacteria. Darby et al. (Darby R. T. and Kaplan A. M., Fungal susceptibility of polyurethanes. Appl. Microbiol., 16, 900-905 (1968)) have performed fungal degradation tests on various polyurethanes. They have reported that ester-based polyurethanes are more sensitive to degradation than ether-based polyurethanes, and that degradation profiles vary depending on the type of isocyanate and/or polyol. Kay et al. (Kay, M. J., McCabe, R. W., Morton, L.H.G., Chemical and physical changes occurring in polyester polyurethane during biodegradation. Int. Biodeterio. Biodegrad., 31, 209-225 (1991)) have isolated 15 bacterial strains capable of degrading ester-based polyurethanes and have also reported the results of degradation profiles examined for Corynebacterium strains having a strong degradation ability.
However, there is almost no knowledge or information about degradation of urethane bonds in polyurethanes. Although some reports indicate that urethane bonds are hydrolyzed during microbial degradation, no clear causal relation has been found between urethane bond cleavage and microorganisms (B. Jansen et al., Evidence for degradation of synthetic polyurethanes by Staphylococcus epidermidis. Zentralbl Bakteriol., 276, 36 (1991); Darby R. T. and Kaplan A. M., Fungal susceptibility of polyurethanes. Appl. Microbiol., 16, 900-905 (1968)).
On the other hand, low molecular urethane compounds are already reported to undergo microbial degradation, and such degradation is known to be catalyzed by esterase. However, most of these reports are directed to improvement of alcohol drinks or degradation/clarification of carbamate insecticides (JP 01-300892 A, JP 01-240179 A, JP 02-128689 A, JP 03-175985 A, JP 04-104784 A, JP 04-325079 A); none of these techniques can be adapted to polyurethane degradation. Fungal degradation is reported as a technique for degrading substances which can be used as source materials for polyurethanes (JP 09-192633 A), but this technique does not use bacteria that can be easily adapted for large scale culture.
In relation to solid polyurethane-degrading bacteria, the following strains are known to degrade polyester-based polyurethanes: Paenibacillus amylolyticus strain TB-13 (Japanese Patent Application No. 2002-334162) and Comamonas acidovorans strain TB-35 (T. Nakajima-Kambe, F. Onuma, N. Kimpara and T. Nakahara, Isolation and characterization of a bacterium which utilizes polyester polyurethane as a sole carbon and nitrogen source. FEMS Microbiology Letters, Vol. 129, 39-42, 1995). However, while these strains do actually degrade ester bonds in urethane, they do not substantially degrade urethane bonds. Thus, to ensure complete bacterial degradation of polyurethanes, there is a demand for bacteria that are capable of degrading urethane bonds.